1. Field of the Invention
The present invention relates to fuel cell systems and methods for controlling fuel cell systems.
2. Description of the Related Art
Fuel cell systems equipped with fuel cells that include anodes, cathodes, and electrolyte membranes therebetween generate power by electrochemical reactions between a fuel gas (e.g., hydrogen) supplied to the anodes and an oxidant gas (e.g., air) supplied to the cathodes through the electrolyte membranes. With this type of fuel cell system, fuel gas remaining in the anodes sometimes permeates the electrolyte membranes and enters into the cathodes during a shutdown period of the system. If such a phenomenon occurs, fuel gas accumulates at the cathodes and in the oxidant gas systems through which the oxidant is normally supplied to or discharged from the cathodes.
In attempt to remedy this problem, Japanese Unexamined Patent Application Publication No. 2007-103023 discloses a fuel cell system including an oxidant gas system having an air compressor for supplying oxidant, in which a shutoff valve is provided upstream of a fuel cell and a pressure regulating valve is provided downstream of the fuel cell. In starting this fuel cell system, oxidant gas is supplied with the pressure regulating valve closed, and then the shut valve is closed when the pressure of the oxidant gas reaches a particular level. As a result, the oxidant gas is enclosed. Then fuel gas in the oxidant gas system (i.e., in the cathode and oxidant gas piping) is consumed on a catalyst of the fuel cell and thereby becomes diluted.
However, if the fuel gas concentration in the oxidant gas system is large, the required pressure of the oxidant gas to accomplish dilution may be excessively high. In such a case, as the load of the air compressor for supplying the oxidant gas increases, the power consumption, driving noise, and vibrations may also undesirably increase.